Alterations in GABAA receptors (GABARs) are likely to play pivotal roles in temporal lobe epilepsy. It is now recognized that changes in specific GABAR subunits could alter the characteristics of inhibition in seizure-prone brain regions without necessarily producing a massive or generalized loss of inhibition. The delta subunit will be the focus of these studies because GABARs that contain this subunit appear to be uniquely plastic, play an important role in GABA-mediated tonic inhibition and may be major biological targets of neurosteroids in some brain regions such as the dentate gyrus. The broad hypothesis is that changes in delta subunit expression and localization could influence seizure susceptibility by altering tonic inhibition and responsiveness to neurosteroids. Changes will be studied in a pilocarpine-treated mouse model of recurrent seizures and following administration of a synthetic neuroactive steroid. A combination of light and electron microscopic immunohistochemical methods will be used to identify changes in expression and localization of the delta subunit and related GABAR subunits that include alpha4 and gamma2. The following questions will be addressed: 1) Does a decrease in delta subunit expression lead to alterations in the subcellular localization of other GABAR subunits such as gamma2 in a mouse model of epilepsy? 2) What are the morphological and functional characteristics of interneurons that express high levels of the delta subunit in normal and epileptic animals? 3) What GABAR subunits are localized with the delta subunit in neurons of the dentate gyrus? 4) Is delta subunit expression altered by chronic administration of the neuroactive steroid ganaxolone? In all studies, emphasis will be placed on evaluating potential differences between GABAR subunit changes in principal cells and interneurons in the hippocampal formation. The morphological findings will be related to physiological and biochemical studies of GABARs in other components of this program project. The studies could identify new types of GABAR plasticity that could contribute to seizure activity and might also suggest cell-specific targets for pharmacological treatment of epilepsy and related disorders.